Optimum multiplier phototube bias for low noise



Nov. 17, 1959 I. P. QIYRODMAN 2,913,585

OPTIMUM MULTIPLIER PHOTOTUBE BIAS FOR LOW NOISE Filed Aug. 6, 1957 l l I l "VVVVV- 24 /5 X I 1/6 /6 S/GIVAZ 0U 7'Pl T I INVENTOR.

ISAAC 1. PUD/VAN ATTORNEYS OPTIlVIUM -MULTIPLIER PHOTOTUBE BIAS FOR LOW NOISE Application August 6, 1957, Serial No. 676,693

1 Claim. 01. 250-407 This invention relates to multiplier phototube circuits and more particularly to multiplier phototube circuits having a higher potential bias between the photocathode and the first dynode than between the other dynodes for providing the minimum ambient light noise multiplication through the circuit.

In most known multiplier phototube circuits a lower bias voltage is applied between the photocathode and the first dynode than the bias between the remaining dynodes. This low cathode-to-first-dynode voltage will minimize the dark current noise which is created within the tube as by thermionic emission, ohmic leakage, light feedback, and the like, whether light signals are impinging the photocathode or not. The sensitivity of a multiplier phototube varies with the bias potential between the photocathode and the first dynode which causes an increase in dark current" noises proportional to the increase of this bias potential. In known multiplier phototube circuits where .the sensitivity is increased the phototube may be cooled to reduce the thermionic emission contribution to the dark current noise. In the past the dark current noise was of most concern and in one knowncircuit thelight signals were chopped, converting the light signals to alternating current in the output which could be separated from the dark current noises.

In the present invention a multiplier phototube having a plurality of 'dynodes is used in a circuit to receive intelligence optical light signals together with ambient light signals, the latter of which amounts to noise in the light signals. Such use may be made of this invention asshown and described in the patent application of which I am a joint applicant with James L. Winget, filed on August 5, 1957, and bearing Serial No. 676,450, to a transceiver that has a multiplier phototube as a detector of light signals. It is the ambient light noise which is of most concern in the present invention. It has been found that the output of the multiplier phototube for a received intelligence optical signal could be increased directly with the increase of potential between the phototube and first dynode, but the output noise also increases proportional to the square root of this increase in potential. In this invention the potential bias between the photocathode and the first dynode is made of such high magnitude sufiicient to provide the optimum signal-to-noise ratio at the output of the phototube in order that the intelligence light signals may be distinguished from the ambient light and dark current noise. It is therefore a general object of this invention to provide a multiplier phototube circuit having a high bias potential between the photocathode and first dynode to optimize the signal-to-noise ratio of the multiplier phototube output.

These and other objects, advantages, features and uses may become more apparent as the description proceeds when read in light of the single figure of drawing illustrating the multiplier phototube circuit in schematic form.

United States Patent Referring more particularly to'the figure of the drawing, there is illustrated a vacuum multiplier phototube 10 with the key 11 thereon. This tube is illustrated as having a cathode 12, an anode 13, and a plurality of dynodes shown particularly herein as dynodes from D1 to D10 although a greater or lesser number of dynodes may be used, as desired, in accordance with commercial tubes of this type. The dynodes D1 to D10 are represented in the drawing as being in numerical order clockwise although the pin numbers for this tube would be in a different numerical order. This numerical order of the dynodes is shown to simplify the circuit coupling from the cathode through the dynodes to the anode. The cathode is coupled to dynode D1 through a parallel network consisting of a resistance 14 and a capacitor 15. Dynode D1 is coupled to dynode D2 through a parallel network consisting of a resistance 16 and a capacitor 15. In like manner each dynode is connected to the next succeeding dynode through a similar parallel network, the reference character 16 in each coupling indicating that the value of each resistance 16 is equal to the value of each other resistance 16. Likewise the capacitors 15 all being equal are identified by the same reference character; The dynode D9 diifers in its coupling to the dynode D10 in that the parallel network 15, 16 is coupled through one winding 17 of a transformer 18, in series, to the dynode 10. The anode 13 is coupled through a transformer winding 19 to a posi tive or fixed voltage source via the conductor 20. The windings 17 and 19 constitute the primary windings of the transformer 18, the secondary winding 21 of which is coupled to a sheathed cable 22 for transmitting the output signal of the circuit. The secondary winding 21 is center tapped and coupled to the fixed potential through the conductor 23 and to the sheathing of the cable 22. The fixed voltage source is also coupled via the conductors 20 and 23 to the common terminal 24 of the parallel network 15 and 16' connecting the winding 17 to the dynode 10.

The cathode 12 is coupled to a negative voltage source through the conductor 25 which negative voltage source is herein shown as minus 1250' volts purely for the purpose of illustration. Dynode D1 is directly connected to a negative voltage source herein illustrated as being minus 1000 volts purely for the purpose of illustration. By this arrangement a potential bias is placed across the photocathode and first dynode to the amount of 250 volts as illustrated herein. The remaining dynodes have approximately volts bias between them with approximately 100 volts between dynode 10 and the anode as illustrated herein. While these voltages are indicated in the drawing purely for the purpose of illustration, it should be understood that other bias voltages may be used as commercial multiplier phototubes may permit or require. The above illustrated voltages were found to be suitable for Dumont multiplier phototubes bearing the identifying numbers 1290 or 6467 although the invention is not limited in any manner to these tubes.

In the above illustration of values it has been found that each resistor 16 of a value approximating 10,000 ohms and each capacitor 15 of a value of .01 microfarad were suitable for the parallel networks. Registrar 14 is of the order of approximately 100,000 ohms since the voltage bias between the photocathode 12 and dynode D1 is more than twice the voltage bias existing between the remainder of the dynodes.

In the operation of the multiplier voltage circuit the high potential bias between the photocathode 12 and first dynode D1 produces considerable dark current within the tube which will likewise produce additional noise at the output of thecable 22 although this dark current noise is negligible in the overall operation of this circuit in the use of the multiplier photocathode tube to pick up intelligence light signals by the impingement thereof .on the photocathode 12. The photocathode 12 .not only becomes excited by intelligence light signals but also .by ambient light signals .which may be produced from sunlight or by the reflection of sunlight on atmospheric particles or objects in the atmosphere. It has been found that the electrical output signals over the cable 22 resulting from intelligence light signals received on .the photocathode 12 vary in a direct proportion to the sensitivity between the photocathode and the first .dynode D1, whereras the ambient light signals produce electrical output signals in the cable 22 in a proportion varying as the square root of this sensitivity. The sensitivity varies with increasing bias wth a rising characteristic having an approximate logarithmic shape. The optimum operating point is at the 'knee of this rising curve. For low voltages, for example, voltages under approximately 100 used as a bias between photocathode 12 and dynode D1, the electrical signals produced at the output resulting from the ambient light signals exciting the photocathode rise rapidly and predominate over the signals produced by the intelligence light signals. As the bias voltage between photocathode 12 and dynode D1 is increased, the output signals resulting from the intelligence light signal impinging the photocathode 12 more rapidly than the ambient noise signals eventually predominate over the output signals resulting from ambient light signals. It has been found that by increasing the bias voltages between the photocathode 12 and first dynode D1 a greater signal-to-noise ratio isaccomplished; however a region of diminishing return for increasing sensitivity is reached. Higher voltage produces no further effect on the tube behavior but increases the possibility of electrical flash-over. In this circuit intelligence light signals and ambient light signals impinging the photocathode 12 excite the photocathode to drive ofli electrons which are multiplied through the plurality of dynodes to the anodeas is well understood in the art, the high bias between the photocathode 12 and the first dynode causing electricaloutput signals on the anode 13 which greatly distinguish the electrical signals produced by the intelligence .light signals from those electrical signals produced by ambient light signals or any dark current signals generated within the signals or any dark current signals generated within the tube 10. The output electrical signals conducted by the sheathed cable 22 may therefore be used in any desirable manner to particularly established the existence and occurrence with respect to time of the intelligence light signals received.

While many modifications and changes may be made in the circuit arrangement of this illustrated invention such as the changing of voltages and multiplying biasing voltages, it is to be understood that I desire to be limited only by the scope of the appended claim.

I claim:

A multiplier phototube having optimum circuit bias for low noise comprising: a multiplier phototube having an anode, a photocathode, and a plurality of dynodes; a direct current voltage supply circuit coupled across said photocathode and said anode, said anode supply having an inductance therein forming one element of an inductive output; parallel connected resistor and capacitor networks coupling each the photocathode and dynodes in sequence, the last two dynodes and the last dynode and anode being coupled throughsaid inductance thereby placing a biasing potential across said dynodes with respect to each other and on said anode with respect to said dynodes and said photocathode; and a bias voltage applied directly from said direct current voltage supply to the first dynode in sequence from said photocathode of a magnitude approximating two and one-half times the magnitude of the biasing potential applied between the remaining dynodes and between said last dynode and anode, said photocathode-to-first dynode potential being sufficient to optimize the signal-to-noise ratio of intelligence optical light signals and ambient light signals impinging said photocathode'whereby electrical signals corresponding to the intelligence optical lightsignals may be distinguished from the electrical signals corresponding to the ambient light signals at the inductive output.

References Cited in the file of this patent UNITED STATES PATENTS Re. 20,545 Jarvis et al. Nov. 2, 1937 2,404,098 Schade July 16, 1946 2,594,703 Wouters Apr. 29, 1952 2,798,165 Neher July 2, 1957 OTHER REFERENCES Du Mont Multiplier Phototubes, Descriptions and Specifications, Allen B. Du Mont Laboratories, Inc., Clifton, N. J., April 1955 -(pages 8-12-relied on). 

